Automatic background control for television systems



p 9, 1941- R. B. DOME 2,255,484

AUTOMATIC BACKGROUND CONTROL FOR TELEVISION SYSTEMS Filed NOV. 30, 1938 His ttorney.

' iatented Sept. 9, 1941 ETE STATES T AUTOMATIC BACKGROUND cou'rnor. ro'a' TELEVISION SYSTEMS Robert B. Dame, Bridgeport, Conm, assignor to General Electric Company, a corporation of v New York Application November 30, 1938, Serial No. 243,134

7 Claims.

My invention relates to television systems and more particularly to a control arrangement for automatically inserting at a selected point in the television system thatcomponent of the picture signal which determines the background brilliancy of the transmitted'picture. While not limited thereto, my invention is particularly suited for use in a television receiver for the automatic reinsertion of the picture signal backfrequency converter stage, intermediatefrequency stages of amplification, a detector, .and an arrangement for separating from the television signal the synchronizing impulses normally transmitted therewith and for developing from such impulses scanning potentials. The output of the detector included in the apparatus 12 may be amplified before being supplied through a condenser 13 to an electron discharge device ground component which previously was removed It for additionalamplification. Th'e'output 'of at some point in the system during the amplifithe amplifier M' is supplied through a condenser cation of the picture signal. I5 to-the-control electrode it of anelectron dis- An object of my invention is to provide in a charge device which may, for example, bev a television system an automatic background concathode ray picture tube IT. The picture tube trol which permits a more economical use 15 has deflecting electrodes l8, l5l-to which are than heretofore possible of the several high voltsupplied through the respective conductors 20, age potential sources required for the operation 2! from the horizontal and vertical oscillators of the electron discharge devices employed. included in the apparatus !2 scanning or de- A further object of my invention is to profleeting potentials. These potentials operate in vide in a television receiver having automatic a manner well-knownin the art. to producea background control an arrangement which enhorizontal and vertical sweep of the picture ables the operation of the cathode element of a modulated cathode stream across the fluorescent cathode ray picture tube, or other electron disscreen provided in the end of the picture tube charge device to which it may be desirable to thereby to reproduce the transmitted picture; supply automatic background potentials, near The overall or background brilliancy of the ground potential thereby dispensing with the picture may be transmitted as a component of necessity of carefully insulating the cathode and the picture signal. This however, requires the its source of energizing current from eachother use throughout the television system ofdirect and from ground for relatively high values of current amplifiers. Amplifiers of this type when potential in the manner heretofore necessary. used in this manner present considerable prob- An additional object of my invention is to lems and have not been found economically provide in a television system an automatic feasible. For this reason, the receivingappabackground control izrWhich the background ratus I2 customarily includes at least one stage effect may be exaggerated to any desired degree of amplification which in its normal operation as an'a'id to improving the contrast of the reremoves the background component, when transproduced picture. mitted, and amplifies only the picture signal The novel features which I believe to be charcomponent. It therefore becomes necessary in .acteristic of my invention are set forth with the television receiver to reinsert in an autoparticularity in the appended claims. My inmatic manner the component of the television .vention itself, however, both as to its organiza- 40 signal which determines the background bril-- tion and the method of operation, together with li'ancy of the transmitted picture. a further objects and advantages thereof, will best Automatic background control is accomplished be understood by reference to the following in my invention in the following manner. A specification taken in connection with the single high resistance 22 is connected between the grid figure of the accompanying drawing which illus- 23 and cathode 24 of the electron discharge aI'n-* trates a television receiver embodying my inplifier M. The picture signal is supplied through vention. the condenser I3 to the grid 23 where acertain Referring more particularly to the drawing, amount of rectification of the picture signal ocmy invention is illustrated as embodying in a curs by the grid 23 and the condenser 13 theretelevision receiver having an input circuit conupon charges to bias the grid 23 with a neganected to an antenna l0 and ground ll. Teletive potential. The bias voltage which is stored vision signals are supplied by the antenna and by the capacitor l3 leaks on :by way of the ground to receiving apparatus, represented conresistor 22 (the time constant of the condenser ventionally by the rectangle 12, which may comand resistor being made approximately one-fifth prise radio frequency stages of amplification, a of a second) and the magnitude of the biasvc'ltage thus varies in accordance with the overall or background brilliancy of the transmitted picture. The picture signal which is supplied to the grid 23 has a maximum amplitude for white and a minimum amplitude for black. The grid 23 therefore has a relatively large negative bias when a white picture is transmitted and a relatively small negative bias during the transmis sion of a black picture. The average space current of the amplifier M varies in accordance with the bias on the grid 23 and consequently decreases when a white picture is transmitted. This space current fiows through an impedance, which may be comprised by a resistor 25 and parallel connected condenser 25, connected between the cathode 24 and ground. It is evident that the potential appearing across the resistor 25 is large during the transmission of a black picture and is relatively small for a white picture.

The potential drop across the resistor 25 has a polarity as indicated on the drawing. This potential is impressed between the cathode 21 and the control electrode l6 of the cathode ray tube through a circuit which includes the resistor 29 and the left hand portion of the resistor 28. The bias on the control electrode I is determined by the potential drop across the resistor at any time added with proper polarity to that across that portion of the resistor 28 to the left of the manually adjusted contact 30. It will be evident that a black picture produces a large potential drop across the resistor 25 and therefore biases the control electrode l5 negatively so that the average brightness of the picture reproduced on the cathode ray screen decreases as is proper for a black picture. A white picture, on the other hand, produces a much smaller potential across the resistor 25 thereby to allow the potential drop across the resistor 28 to bias the control electrode l6 more positively to increase the average brilliancy of the reproduced picture as is proper for a white picture. In this manner, the average brilliancy or background of the reproduced picture is automatically controlled in accordance with the overall or background brilliancy of the transmitted picture. The resistor 28 provides an initial adjustment on the picture brilliancy in order that the proper range of control may be obtained. This is effectively a manual background control.

My invention has several important advantages. The cathode 21 is operated near ground potential, and therefore requires only a minimum of insulation from ground for itself and associated circuit as well as for the source of current which is used to energize the cathode. An equally important advantage of my invention is that the operation of the cathode 27 near ground potential enables the most economical use possible to be made of the several high voltage sources of potential which are required for the electron discharge devices employed in the television receiver. That is, the source of anode potential required for the amplifier tubes may be connected in series with the source of relatively high potential required for the several anodes of the cathode ray picture tubes.

The exact manner by which the several sources of anode potential may be serially connected is disclosed and claimed in my patent application No. 237,785 filed Oct. 29, 1938, entitled High voltage supply circuit for television systems and assigned to the same assignee as the present in- Vention.

This arrangement consists, briefly, in providing a secondary winding 3| in a power transformer 32, whose primary 33 is connected to a power source and in connecting the secondary winding to a full wave rectifier 35 whose output charges a condenser 35 through a suitable filter 35. The condenser 35 is charged to a potential of 200 to 300 volts to provide a source of anode potential for the amplifier l4 and for similar amplifier tubes included in the apparatus I2. The transformer winding 3| is also connected. through the condensers 37 and 53 to a half wave rectifier 39. Current flows through the rectifier 39 during one-half cycle of the potential supplied by the transformer winding 3| to charge the condensers 37 and 38 substantially to the peak value of the secondary winding potential. The anode of rectifier 39 which is negative with respect to its cathode is connected through the resistance 4| to the positive terminal of the rectifier 34, thus causing an addition of potentials to appear across the capacitor 40. This, for example, may be of the order of 1,000 volts. The series circuit by which this additive voltage is attained may be traced from the grounded terminal of the condenser 40 through condenser 35, filter 35, a resistor El, condenser 38, transformer winding 3|, condenser 31, and a resistor 42, to the ungrounded terminal of the condenser 40.

The potential appearing across the condenser is supplied through a conductor 43 and through the voltage dividers 44, 45 and 45 to the deflecting electrodes |8, l9 and the focusing electrode 41 which are provided in the cathode ray picture tube ii. The voltage dividers 44 and 55 have manually adjusted contacts 48 and 49, respectively, by which the high voltage potential supplied through the conductor 43 to the deflecting electrodes l8, l9 may be controlled to center the picture on the fluorescent screen provided in the end of the picture tube H. The voltage divider 46 has a manually adjustable contact 50 by which the potential supplied to the focusing electrode 4'! may be controlled for the purpose of controlling the size of the illuminated spot produced by the electron stream when it strikes the fluorescent screen.

The anode potential for the amplifier M is supplied through a resistor 5|. In practice, the value of the resistor 5| is generally made approximately equal to the value of the resistor 25 in order that the direct current and alternating current amplifier characteristics of the amplifier M are substantially equal. A circuit embodying my invention may, however, be adjusted to yield exaggerated background effects by making the value of the resistor 25 larger than that of the resistor 5|. The background effects may be minimized by making the value of the resistor 5| larger than that of the resistor 25. Under some conditions of operation, the exaggerated background effects may be desirable as an aid to improving the contrast of the reproduced picture.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications may be made in the circuit elements and their arrangement, and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in a television receiver having automatic background control, of a having one terminal connected through a 'pic-.

source of picture signals, means for amplifying said signals, said means including an electron discharge device having acathode and an anode and two impedances connected therebetween,

one of said impedances being adapted to'develop thereonhigh frequency voltage representing the received picture and the other having substantially zero impedance for high frequency currents representing said picture and adapted to develop thereon voltages varying only with the backa ground brilliance of the transmitted picture, a

picture reproducing devicehaving a cathode and control electrode, a path impenetrable to unidirectional current extending from said control electrode through both of said impedances to the cathode of said picture reproducing device, and a unidirectional current conducting path extending from said control electrode through the last mentioned of said impedances, but excluding the first mentioned of said impedances, to said cathode, the said last mentioned impedance being poled in said last mentioned path to provide a bias on said picture reproducing device varying with said background brilliance.

2. The combination, in a television receiver, of an electron discharge amplifier, an electron discharge picture reproducing device, said amplifier and device each having a cathode, an anode and a control electrode, a source of picture signals and background signals connected between the grid and cathods of said amplifier, a coupling impedance between the anode of said amplifier and ground adapted to develop thereon potentials in accordance with said picture signals, means to supply said potentials to the grid of said device, a resistance connected between ground and both of said cathodes and by-passed for picture signals, means to produce on said resistance a potential variable with said background signals, and a bias connection exclusive of said first mentioned coupling impedance between the grid of said device and ground whereby the potential on said resistance is supplied between the grid and cathode of said device as a bias.

3. The combination, in a television receiver, of an amplifier having an anode, a cathode, and a control electrode, an electron discharge picture reproducing device having a control electrode, a cathode, and a high potential electrode, a source of operating potential for said amplifier having one terminal connected through a picture signal coupling impedance to the anode of said amplifier and its opposite terminal connected to both of said cathodes through a background signal coupling impedance, said high potential elec trode being connected through a source of operating potential and said background coupling impedance to both of said cathodes, a unidirectional current conducting path between said control electrode and cathode of said picture reproducing device including said background signal coupling impedance, means to supply picture si nals from said picture signal coupling impedance between said control electrode and cathode of said reproducing device, and means to produce both picture signals and background signals on the control electrode of said amplifier whereby said signals are amplified thereby and supplied between the control electrode and cathode of said picture reproducing device.

4. The combination, in a television receiver of an amplifier having an anode, a cathode, and a control electrode, an electron discharge picture reproducing device having a cathode and a con- 7 trol electrode, a source of operating potential ture signal coupling impedance to said anode of said amplifier and its opposite terminal connected through a background coupling 'impedance to both of said cathodes, means to main- 7 tain said opposite terminal and said control electrode of said picture reproducing device at substantially fixed unidirectional potential while said cathodes vary in potential by reason of variations in potential on said background coupling impedance, a connection for picture signals between the anode of said amplifier and the control electrode of said picture reproducing device, and means to supply both picture signals-and background signals between' the control electrode and cathode of said amplifier whereby they are amplified and said picture'signals are supplied between said terminal at fixed unidirectional po tential and the control electrode of said picture reproducing device and said background signals are supplied between said terminal at fixed unidirectional potential and the cathode of picture reproducing device.

5. The combination, in a television receiver,

of an electron discharge amplifier having an anode, a cathode, and a control electrode, a picture reproducing discharge device having a cathode, control electrode and a high potential 'electrode, a source of energizing potential having one terminal thereof connected to both of said cathodes through a resistance and other terminals thereof positive with respect to said one terminal connected respectively to said anode through an impedance and to said high potential electrode, means to by-pass said resistance with respect to picture currents having frequencies higher than variations in average brilliance of the transmitted picture, a control electrode circuit for said amplifier adapted to have impressed thereon the received television signal and includ ing means to produce a voltage on said grid which varies negatively with respect to the oathode in accordance with the average brilliance of '7 the transmitted picture whereby the voltage on said resistance varies with said average brilliance of said picture, a bias connection for unidirec-' tional currents from the control electrode of said picture reproducing device to the negative terminal of said resistance, and means to supply picture signal electromotive force existing between said negative terminal of said resistance and said anode to said control electrode of said picture reproducing device.

6. The combination in a television receiver, of an amplifier having a cathode, a control electrode, and an anode, a picture reproducing de vice having a cathode, and a control electrode, and means to transmit both picture and background signals through said amplifier while operating said cathodes at substantially the same unidirectional potential, said means comprising means connecting said cathodes together for unidirectional currents and to ground through a background coupling impedance, a picture signal coupling impedance connected between said anode of said amplifier and ground whereby said coupling impedances are in series between the anode and cathode of said amplifier, means connecting the control electrode of said picture reproducing device to said anode of said amplifier for picture signals and to ground for unidirectional current, and means to supply alternating picture signals and negative bias potential varying with the average intensity thereof to the control electrode of said amplifier wherebythe potential between the control electrode and cathode of said picture reproducing device varies with both picture and background signals, and said cathodes vary from ground only by the potential on said background coupling impedance.

7. The combination, in a television receiver, of a picture reproducing device having a high potential electrode, a control electrode and a cathode, an amplifier having a cathode, a control electrode and an anode, a source of unidirectional operating potential having its negative terminal connected to both of said cathodes through a background signal coupling impedance and to the control electrode of said picture reproducing device through a unidirectional current path,

and having its positive terminal connected to said high potential electrode, additional picture signal coupling means between said anode of said amplifier and the control electrode of said picture reproducing device, means to supply picture signals to said control electrode of said amplifier for reproduction by said picture reproducing device, and means to produce on said control electrode of said amplifier a bias varying negatively with respect to said cathode in accordance with background signals thereby to vary the potential on said background coupling impedance and between the cathode and control electrode of said picture reproducing device.

ROBERT B. DOME. 

